Polymer electrolyte fuel cells (PEFC) have a stacked structure constituted by a plurality of single cells that exert a power generation function. Each of the single cells has a membrane electrode assembly (MEA) including (I) a polymer electrolyte membrane, (II) a pair of a anode catalyst layer and a cathode catalyst layer (electrode catalyst layers) that interpose the polymer electrolyte membrane therebetween, and (III) a pair of an anode gas diffusion layer and a cathode gas diffusion layer that interpose the electrode catalyst layers therebetween and disperse supply gas. The MEA in one single cell is electrically connected to another MEA in the adjacent single cell via a separator. Thus, a fuel cell stack is constituted by the single cells that are stacked on top of each other.
The fuel cell stack described above functions as a power generation means available for various purposes. In the fuel cell stack, the separator functions to electrically connect the adjacent single cells to each other as described above. In addition, the surface of the separator facing the MEA is generally provided with gas flow paths. The gas flow paths function as gas supply means to supply fuel gas and oxidant gas to an anode and a cathode, respectively.
The following is a brief explanation of a power generation mechanism of the PEFC. During the operation of the PEFC, fuel gas (such as hydrogen gas) is supplied to an anode side of the single cells, and oxidant gas (such as air and oxygen) is supplied to a cathode side. As a result, electrochemical reactions represented by the following reaction formulae proceed at the anode side and the cathode side, respectively, so as to generate electricity.Anode side: H2→2H++2e−  (1)Cathode side: 2H++2e−+(½)O2→H2O  (2)
A separator constituted by metal has relatively high intensity compared with a carbon separator and an electrical conductive resin separator. Therefore, the thickness of the metal separator can be reduced to some extent. In addition, since the metal separator has excellent electrical conductivity, there is an advantage of reducing contact resistance to the MEA. However, the metal separator has a possibility of a decrease in electrical conductivity caused by corrosion derived from produced water and a potential difference caused during the operation, and a possibility of a reduction in power in the stack in association with the decrease in electrical conductivity. Therefore, the metal separator is required to have improved resistance to corrosion while the excellent electrical conductivity is ensured.
There has been a known method of forming an oxide film on a substrate of a metal separator so as to be provided between the substrate and an electrical conductive thin film (for example, Patent Document 1). According to such a method, it is possible to obtain a separator for a fuel cell that can ensure electrical conductivity and suppress dissolution of metal constituting the substrate and has excellent durability. In addition, Patent Document 1 discloses that an intermediate layer for improvement in adhesion is provided between the oxide film of the substrate and the electrical conductive thin film. Thus, adhesion between the oxide film of the substrate and the electrical conductive thin film can be improved. Here, the intermediate layer is prepared by a sputtering method.